1. Field of the Invention
This invention relates generally to integrated circuit (IC) current references, and more particularly, to IC current references which provide reference currents for use both internal and external to an IC.
2. Description of the Related Art
Integrated circuits (ICs) often require the use of a reference current. Such currents are used, for example, as a reference for a current limit detection circuit, to generate bias currents, or for any of a number of other purposes.
A reference current is typically generated by applying a reference voltage generated internally to the IC across the terminals of an internal resistor. Such an arrangement is illustrated in FIG. 1. Here, an IC 10 receives a reference voltage Vref from a voltage source (not shown). Vref is buffered with an amplifier 12, and the buffered output 14 is used to drive a transistor 16, shown here as an NMOS FET, though other transistor types could also be used. An internal resistor 18 having a resistance R is connected between the source of FET 16 and one of the IC's input/output (I/O) pins 20, and the FET's drain is connected to the input 22 of a current mirror 24 which is referred to a positive supply voltage V+. I/O pin 20 would typically be connected to a circuit common point 26.
In operation, Vref drives FET 16 to conduct a current Iref which is given by Vref/R. Current mirror 24 then produces copies (28) of Iref, which can be used for other circuits internal to IC 10 as needed.
Ideally, resistor 18 is a high quality resistor having a low temperature coefficient (TC) and voltage coefficient (VC) and a known resistance value. For example, some IC processes allow the fabrication of thin film resistors, which have both low TC and VC and may have an initial accuracy of 15% or better. However, for some low cost IC processes, thin film resistors are not available, and other types such as polysilicon or diffused resistors must be used. In these cases, initial accuracy may be no better than 30% or more and TCs can be on the order of 1000 ppm/° C. or larger, leading to unacceptably large errors over temperature. If a higher accuracy reference current is needed, errors due to the internal resistor must be corrected and/or compensated for using correction circuits, on-die trimming, additional test time, external circuitry requiring additional pin count, etc. Expensive laser or fuse-blow trimming can improve initial accuracy, but have little to no effect on the internal resistor's non-ideal temperature and voltage characteristics.
Reference voltages and currents generated within an IC are often used by other circuitry which is external to the IC. For example, in FIG. 1, IC 10 is used to provide regulation for a negative supply voltage channel V−. A negative voltage is often generated by means of a charge pump; in FIG. 1, some of the charge pump circuitry (30a) is internal to the IC (such as switches, control circuitry, etc.) and some (30b) is external (feedback resistors, switches, capacitors, etc.), with the internal and external portions coupled together via an I/O pin 31. Regulation is provided by means of an error amplifier 32, one input of which is connected to circuit common point 26 and the other of which is connected to an I/O pin 34. Pin 34 is connected to the junction 36 of two resistors, one of which (38) is connected between junction 36 and V−, and the other of which (40) is connected between junction 36 and Vref via an I/O pin 42. Resistors 38 and 40 are chosen such that the voltages at the inputs of error amplifier 32 are equal when V− is at a desired value. While the operation of the regulation scheme shown in FIG. 1 is satisfactory, it does require the use of three I/O pins (31, 34, 42) and two resistors (38,40).
Extreme cost pressures on some ICs push manufacturers to use inexpensive IC processes and lower pin count packaging. But as noted above, these inexpensive processes typically lack high quality resistors, so that an internally generated reference current may be unacceptably inaccurate. And a limited pin count may make an application of a high quality current reference for internal bias generation or current limit detection impractical.